I recall seeing a significant scar on my mother’s arm as a child. It sits high up, close to her shoulder, and resembles a ring of little indents in her skin surrounding a larger depression. Don’t ask me why that caught my attention all those years ago; I can’t remember. I just remember that it existed, but as is often the case, I forgot about it over the years.

Obviously, I didn’t forget it existed (it’s still in the same position), but I did forget that I was once interested by what had created it. Maybe I asked my mother once, and she explained. If she did, I forgot about it as well. That was until I assisted an elderly woman off a train one summer a few years ago, and I happened to see the identical scar, in the same location, as my mother’s. Needless to say, my curiosity was piqued, but with the train about to depart for my destination, I couldn’t exactly question her about the origins of her scar.

Instead, I called my mother, who disclosed that she had informed me more than once – apparently, my brain didn’t think the answer was significant enough to remember – and that her scar was the result of the renowned smallpox vaccination. Smallpox is a viral, contagious illness that previously frightened humans. It causes a severe skin rash and fever, and during the most severe outbreaks in the twentieth century, the Centers for Disease Control and Prevention estimate that three out of every ten sufferers died. Many more patients were left disfigured.

The smallpox virus was proclaimed “extinct” in the United States in 1952 as a result of the vaccine’s successful and broad adoption. Smallpox vaccines were really phased out of standard immunizations in 1972. Up until the early 1970s, however, all children were immunized against smallpox, and the vaccines left a distinct imprint. Consider it the first vaccine passport: a scar indicating that you had been effectively inoculated against smallpox.

And, yes, you guessed it, my mother bears that same scar (as do almost all others her age). Why did the smallpox vaccine leave a scar? The smallpox vaccine left scars owing to the body’s healing mechanism. The vaccination was administered using a specific two-pronged needle, which differed from the way many other vaccines are supplied today.

To transfer the vaccine to the skin’s dermis (the layer beneath the epidermis), the person providing the vaccination made many punctures in the skin (rather than just one as is typical with modern immunizations). The virus in the vaccine then began to replicate, causing spherical lumps to form. The bumps subsequently evolved into vesicles (small, fluid-filled blisters) that would eventually burst and scab over.

The consequence is the infamous scar discussed in this article. Are you old enough to have a scar from the smallpox vaccination? Please let us know in the comments.